The application of ultrasonics to the analysis of material properties is constrained by physics and instrumentation. Advances in the application of ultrasound analysis have come through improvements to transducer sensitivity, focused devices and attempts to better match the transducer to the media being analyzed.
Using conventional techniques, the application of air-coupled ultrasonics has generally been regarded as impractical with a few specific exceptions. However, many applications where contact methods are not permitted are possible with air-coupled ultrasonics, such as the analysis of green ceramics, paper products and food products.
Conventional ultrasound testing is done using one of three modes of operation. The first mode uses a device known as a pulser to produce a high power impulse that is applied to an ultrasound transducer. An ultrasound pulse is then directed at a test piece. Typically, a transmitted or reflected pulse is captured and analyzed to determine the time of flight in the test piece. The second mode of operation involves continuous waves whereby a single frequency is applied to an ultrasound transducer. Again, a transmitted or reflected pulse is captured. The applied single frequency waveform is used as a reference to compare phase and amplitude of the captured ultrasound signals. The third mode of operation is a hybrid called a tone burst whereby a finite duration ultrasound signal is generated at a specific frequency. Tone burst signals are used in applications where stepped frequency response is desired or in pulse-echo applications where an infinite wave precludes using the same transducer to transmit and receive ultrasound.
There exist constraints on the amount of energy that can be used for ultrasonics. Any transducer has physical limits that cannot be exceeded without damaging the transducer itself. Furthermore, each transducer has a range over which it can produce an output that is linearly proportional to an input voltage within some frequency band. If this linear range is exceeded, the analysis of any captured or received signal is complicated by the non-linear nature of the sound source. The medium used to propagate sound waves, whether gas, liquid or solid, also has a finite limit to the linear sound pressure levels that can be sustained. In the case of solid materials, it is possible to damage or alter the material with the ultrasound which defeats the purpose of non-destructive testing.
With ultrasonic pulsers, a short duration voltage spike is presented to an ultrasound transducer and the resulting sound waves are propagated through the test piece or medium to a receiving transducer. Since a transducer is limited in the amount of energy that can be delivered in a single pulse, an upper limit exists on the attainable signal-to-noise ratio. In addition, as a pulse propagates through any medium, it is attenuated by the physical properties of the medium and by geometric factors related to beam spreading. As the distance traversed by a pulse increases, the amplitude of the received signal decreases.
Continuous wave systems typically use a lock-in amplifier to detect very weak signals while preserving the phase and amplitude information. These systems work very well in a laboratory setting, but due to the trade-off between integration time and sensitivity, they are limited to slow speed applications.
Tone burst systems have been employed to study ultrasonic resonance peaks associated with physical geometry. The concept involves stepping the frequency of a tone burst system to look for specific frequencies at which the amplitude is maximized or minimized. Tone burst methods can be applied to preserve phase and amplitude, however the time required to cover a useful frequency spectrum in detail can be prohibitive in practical applications. A resonance technique can provide information about thickness and velocity of a test sample provided the geometry is fairly simple and the internal structure is well known.
A method developed in other fields of study comprises an application of a time domain filter coupled with a specifically constructed waveform to yield a desired impulse response within the bandwidth limitations of the transducers. A wide bandwidth signal that meets specific criteria is constructed and used to synthesize an impulse response. The duration of the signal is arbitrary and does not affect the resolution of the system, therefore, a very long waveform representing very large total energy can be employed to compensate for boundary reflections, attenuation and geometric losses. We refer to a system using such a method as a synthetic impulse (or SI) system. An SI system preserves the phase of the waveform. The phase angle of the synthetic impulse can be used to precisely determine the time of flight to a tiny fraction of a wavelength. SI images can distinguish between two or more synthetic impulses by how the waveforms are coded. Independently coded waveforms make a number of applications possible.
It is a common misconception that improving time resolution necessarily implies increasing frequency. However, the time resolution of any signal is limited by the absolute bandwidth of the signal and not the frequency. Larger bandwidths generate improved time resolution. Attenuation in the propagation of ultrasound generally increases with frequency. An ideal ultrasonic analysis system should use broad bandwidth transducers at lower frequencies to improve time resolution while reducing losses.
It is an object of the present invention to provide methods of greatly enhancing the dynamic range, sensitivity and accuracy of ultrasonic measurements of material properties (either solid, liquid or gas) using ultrasonics in air-coupled, liquid-coupled or dry-coupled modes.
It is another object of the present invention to provide ultrasonic testing methods and apparatus using specially constructed waveforms and a time domain filter coupled to wide-band ultrasonic transducers.
It is a further object of the present invention to extract ultrasonic spectroscopic information about test materials in a fashion that separates the geometric properties from the acoustic properties.
It is yet another object of the present invention to provide means of compensating for air column losses in ultrasonic spectroscopy applications using information derived from synthetic impulse measurements.
It is the further object of the present invention to improve image quality in the Synthetic Aperture Focussing Technique (or SAFT).
It is the further object of the present invention to provide radial resolution with phased arrays to achieve angular resolution, thus forming two-dimensional images.
It is yet another object of the present invention to provide a practical method of compensating for air column fluctuations in air-coupled applications.
This invention relates generally to methods of creating, transmitting, receiving and processing ultrasound to enhance dynamic range, sensitivity and accuracy of measurement of ultrasonic signals. SI techniques are applied to contact ultrasonics and non-contact ultrasonics to boost sensitivity and dynamic range. The invention can be used to extend the capabilities of synthetic aperture focusing techniques and imaging methods based on array transducers.
Briefly, according to one embodiment of this invention, there is provided a method of measuring material properties using ultrasound comprising the steps for:
a) generating a wide-band signal having a varying frequency waveform of fixed but arbitrary duration;
b) storing a replica of the wide-band signal;
c) applying the wide-band signal to an ultrasound transducer and directing the ultrasound generated by the transducer at a test piece;
d) capturing a received signal being the ultrasound transmitted through or reflected from the test piece with an ultrasound transducer; and
e) convolving the replica and the received signal to form one or more synthetic impulse images.
In another embodiment of this invention, there is provided a method of extracting ultrasound spectroscopic information from a test piece comprising the additional step of performing a Fourier transform on the synthetic impulse image and comparing it to the frequency spectrum of the wide-band signal. Preferably, air column error is determined by repeating the process with the specimen removed and the transducers moved closer together by the thickness of the specimen or by calculating the air column error.
According to yet another embodiment of this invention, there is provided an improved process for producing an image using the SAFT process comprising the steps for each individual reading required by the SAFT process comprising generating synthetic impulse images for each position at which a reading is taken.
According to yet another embodiment of this invention, there is provided a method of producing a two-dimensional image using an array of ultrasound transducers comprising the steps,
for at least one transducer:
a) generating a wide-band signal having a varying frequency waveform of fixed but arbitrary duration;
b) storing a replica of the wide-band signal; and
c) applying the wide-band signal to at least one ultrasound transducer and directing the ultrasound generated by the transducer at a test piece;
for a plurality of transducers in the array:
d) capturing a received signal being the ultrasound transmitted or reflected from the test piece;
e) convolving the replica and the received signal to form one or more synthetic impulse images; and
f) displaying a two-dimensional image based upon the synthetic impulse images and the coordinates of the transducers in the array.
According to a still further embodiment of this invention, there is a method of making air-coupled ultrasound measurements on a test piece comprising the steps for:
a) generating a wide-band signal having a varying frequency waveform of fixed duration;
b) storing a replica of the wide-band signal;
c) applying the wide-band signal to an ultrasound transducer and directing the ultrasound generated by the transducer at a test piece;
d) capturing a received signal being the ultrasound transmitted through the test piece with an ultrasound transducer;
e) convolving the replica and the received signal to form one or more synthetic impulse images;
f) removing the test piece and moving the transducers closer together by the thickness of the test piece and repeating steps a) to e) to determine the attenuating effects of the air column and instrumentation; and
g) calculating the ratio of the specimen spectrum to the air column spectrum.
According to yet another embodiment, there is provided a method of making air-coupled ultrasound measurements on a test piece comprising the steps for:
a) generating first and second different wide-band signals having a varying frequency waveform of fixed duration;
b) storing a replica of each of the wide-band signals;
c) applying the wide-band signals to spaced ultrasound transducers and directing the ultrasound generated by the transducers at a test piece;
d) capturing a received signal being the ultrasound transmitted through the test piece and reflected signals returned from each surface of the test piece to respective transducers;
e) convolving each of the replicas and the received signals to form synthetic impulse images for transmitted signals and signals reflected from each surface of the test piece;
f) determining the air column attenuation from the synthetic impulses generated with the reflected signals; and
g) calculating the ratio of the specimen spectrum to the air column spectrum.
Synthetic impulse output waveforms are constructed to distinguish one impulse from another. It is possible to construct an infinite number of different SI output waveforms that will exhibit almost no overlap unless the waveforms are identical. This approximates orthogonality in functional analysis. By creating two or more separate waveforms that are approximately orthogonal to one another, it is possible to recover two or more independent images concurrently. By choosing appropriate transducer arrangements, it is possible to excite different modes of vibration in a sample at the same time and then to process each mode separately. An example of separate SI waveforms is a first frequency ramped chirp running from low frequency to high frequency and a second orthogonal waveform being the frequency ramped chirp running from high frequency to low frequency.